Method and apparatus for measuring a toner concentration of a two-component developer

ABSTRACT

In an electrophotographic or electrostatic image forming device, the toner concentration of a two-component developer, i.e., a mixture of toner and carrier is measured on the basis of an output of a magnetic bridge type concentration sensor responsive to the magnetic permeability of the developer. The sensor has a surface whose mean roughness along the center line is less than 0.4  mu m Ra.

BACKGROUND OF THE INVENTION

The present invention relates to a method of measuring the concentrationof toner contained in a two-component developer applicable to adeveloping device of an electrophotographic electrostatic or imageforming apparatus, and an apparatus therefor.

To develop a latent image electrostatically formed on an image carrier,use is made of either a one-component developer, i.e., toner or atwo-component developer which is a mixture of toner and carrier.Regarding the two-component developer, toner and carrier are eachcharged to a particular polarity by being agitated, i.e., by friction.The charged toner develops a latent image, or electrostatic chargeimage, having been charged to the opposite polarity to the toner. Inthis connection, in reversal development, the toner deposits on exposedportions. There are available a magnet brush method using iron powder ascarrier, a cascade method using beads as carrier, a fur brush method andso forth, which are selectively used in matching relation to the kindsof toner and carrier.

The management of the toner concentration of the developer is vital ininsuring high quality and stable images. The toner concentration of thedeveloper is measured by, for example, a magnetic bridge type sensorwhich is responsive to the magnetic permeability of the developer. Atoner concentration is estimated on the basis of a relation between atoner concentration and a sensor output and then fed back. This kind ofscheme is disclosed in, for example, Japanese Patent Laid-OpenPublication No. 4-24651. However, when the fluidity of the developerstored in a developing device decreases, the developer cannot beagitated stably. As a result, the sensor fails to operate stably and, inthe worst case, outputs a signal not representing the actual tonerconcentration.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodcapable of measuring the toner concentration of a two-componentdeveloper accurately, and an apparatus therefor.

In accordance with the present invention, in a method of measuring thetoner concentration of a two-component developer by causing it to passthrough a sensor portion included in a magnetic bridge typeconcentration sensor and in terms of magnetic permeability, the sensorhas a surface whose mean roughness along the center line is less than0.4 μm Ra.

Also, in accordance with the present invention, an apparatus formeasuring the toner concentration of a two-component developer has amagnetic bridge type concentration sensor for sensing the magneticpermeability of the developer by causing the developer to pass through asensor portion of the sensor, and a measuring device for measuring atoner concentration on the basis of the magnetic permeability sensed bythe sensor. The sensor has a surface whose mean roughness along thecenter line is less than 0.4 μm Ra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription taken with the accompanying drawings in which:

FIG. 1 is a section of a developing device with which the presentinvention is practicable;

FIG. 2 is a block diagram schematically showing a magnetic bridge typesensor included in the developing device of FIG. 1; and

FIG. 3 is a graph indicating a relation between toner concentration andoutput voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, experiments have shown that adevice for measuring the toner concentration of a two-componentdeveloper, preferably a magnetic bridge type sensor, should preferablyhave a mean surface roughness of less than 0.4 μm along the center linethereof. In this connection, a conventional sensor of the kind concernedhas a mean surface roughness of 0.4 μm to 0.6 μm Ra along the centerline. Such a mean surface roughness equal to or greater than 0.4 μm Raincreases the coefficient of friction between the sensor surface and thedeveloper, so that it takes a substantial period of time for thedeveloper to begin to flow on the sensor surface smoothly. As a result,the sensor output is not stable in the initial stage. The term "Ra"designates the center line average roughness, as described in JIS B-0601(1994).

On the other hand, in accordance with the present invention, thedeveloper passing through the sensor portion of the sensor shouldpreferably have a volume density of 1.9 g/cm³ to 2.3 g/cm³ when thetoner concentration is 2.0 wt %. Volume density greater than 2.3 g/cm³apparently reduces the volume of the developer and thereby degrades theagitation of the developer. As a result, it takes a substantial periodof time for the developer to begin to flow on the sensor surfacesmoothly, preventing the sensor output from becoming stable in theinitial stage. Conversely, volume density smaller than 1.9 g/cm³apparently increases the volume of the developer and is apt to cause thedeveloping device to overflow. The volume density of the developer hasconventionally been 1.8 g/cm³ to 1.9 g/cm³ in the case of ferritecarrier or 2.5 g/cm³ to 4.0 g/cm³ in the case of iron powder carrier.

More desirably, the developer passing through the sensor portion shouldhave a carrier volt, me density ranging from 1.5 g/cm³ to 3.1 g/cm³.Carrier volume density greater than 3.1 g/cm³ apparently reduces thevolume of the developer and thereby degrades the agitation of thedeveloper. As a result, it takes a substantial period of time for thedeveloper to begin to flow on the sensor surface smoothly, preventingthe sensor output from becoming stable in the initial stage. Carriervolume density smaller than 1.5 g/cm³ apparently increases the volume ofthe developer and is apt to cause the developing device to overflow.

It is more preferable that the toner of the developer passing throughthe sensor portion of the sensor has a cohesion degree of 5% to 40%. Inthis connection, toner in general has a cohesion degree of 5% to 10%.Cohesion degrees higher than 40% degrade the agitation of the developer.As a result, it takes a substantial period of time for the developer tobegin to flow on the sensor surface smoothly, preventing the sensoroutput from becoming stable in the initial stage. Conversely, cohesiondegrees lower than 5% cause an excessive amount of toner to bereplenished into the developing device. This makes it difficult todeposit a desired amount of charge on the toner and brings about, forexample background contamination due to an increase in tonerconcentration.

Various kinds of toner replenishing methods have been proposed in thepast. A large capacity hopper, for example, promotes the effective useof a limited space available in a copier or similar image formingapparatus. In light of this, the large capacity hopper may tie held in ahorizontal position and rotated about its own axis to replenish toner.For this type of toner replenishing method, a toner cohesion degreeranging from 5% to 40% is effective.

The toner cohesion degree can be controlled to a desired degree if asmall amount of hydrophobic silica is added to the toner. Hence, it ispossible to measure toner concentrations accurately without affectingthe sensor output.

In accordance with the present invention, while the volume mean particlesize of the toner passing through the sensor portion of the sensor isnot limited, it should preferably range from 5 μm to 10 μm in respect ofimage density.

In accordance with the present invention, the mean roughness of thesensor surface along the center line was defined according to JapaneseIndustrial Standard (JIS) B-0621 and B-0651, while the volume densitiesof the developer and carrier were measured by a volume density measuringmethod (JIS Z-2504). Further, to measure the cohesion degree of toner, apowder tester available from Hosokawa Micron (Japan) was loaded withscreens of mesh sizes of 150 μm, 75 μm and 45 μm. 2 g of sample tonerwas put in the top screen, and then the powder tester was caused tovibrate for 30 seconds at an amplitude graduation of 1 mm. Assume:##EQU1## The cohesion degree (%) was determined by summing up the above(a), (b) and (c).

Hereinafter will be described toner with which the method of the presentinvention is practicable. A binding resin may be selected from a groupof polymers consisting of styrenes including polystyrene,poly-p-chlorostyrene and polyvinyl toluene and their substitutes; and agroup of styrene copolymers consisting of styrene-p-chlorostyrene,styrene-propylene, styrene-propylene copolymer, styrene-vinyl toluenecopolymer, styrene-vinyl naphthalene copolymer, styrene-methyl acrylatecopolymer, styrene-ethyl acrylate copolymer, styrene-methyl methacrylatecopolymer, styrene-ethyl methacrylate copolymer, styrene-butyl acrylatecopolymer, styrene-octyl acrylate copolymer, styrene-methylmethacrylate, styrene-ethyl methacrylate copolymer, styrene-butylmethacrylate copolymer, styrene-α-methyl chloromethacrylate copolymer,styrene-acrylonitrile copolymer, styrene-vinyl methyl ether copolymer,styrene-vinyl ethyl ether copolymer, styrene-vinyl methylketonecoploymer, styrene-butadien copolymer, styrene-isoprene copolymer,styrene-acrylonitrile-indene copolymer, styrene-maleate copolymer, andstyrene-ester maleate copolymer.

Any of the following resins may be added to the binding resin:polymethyl methacrylate, polybutyl methacrylate, vinyl polychloride,vinyl polyacetate, polyethylene, polypropylene, polyester, polyurethane,polyamid, epoxy resin, polyvinyl butyral, polyacrylate resin, rosin,modified resin, terpene resin, phenol resin, aliphatic or alicyclichydrocarbon resin, aromatic petroleum resin, chlorinated paraffin,paraffin wax, etc.

To fix toner images by pressure, use may preferably be made of thefollowing substances (with or without the above resins added thereto):polyolefin (low molecular weight polyethylene, love molecular weightpolypropylene, polystyrene oxide, poly-4-ethylene fluoride, etc.), epoxyresin, polyester resin, stylene-butadien copolymer (monomer ratio of5-30:95-70), olefin copolymer (ethylene-acrylate copolymer,ethylene-ester acrylate copolymer, ethylene-methacrylate copolymer,ethylene-ester methacrylate, ethylene-vinyl chloride copolymer,ethylene-vinyl acetate copolymer or ionomer resin), polyvinylpyrolydone, methylvinylether-maleic anhydride, malic acid modifiedphenol resin, phenol modified terpene resin, etc.

For a coloring agent, use may be made of any conventional pigments anddyes alone or in combination. Examples are carbon black, lamp black,iron black, ultramarine, Nigrosine dye, aniline blue, phthalocyanineblue, phthalocyanine green, Hansa Yellow G, Calko Oil Blue, chromeyellow, quinacridone, Benzidine Yellow, Rose Bengal, tryarylmethane dye,and monoazo or diazo pigments.

Additives may be added to the toner applicable to the present invention,as needed. The additives include Teflon, zinc stearic acid and otherlubricants, cerium oxide, silicon carbide and other abrasives, aluminumoxide and other fluidity agents, anti-caking agents, carbon black, tinoxide and other conductivity agents, and low molecular weight polyolefinand other fixation promoting agents.

Preferably, the additives should have primary particles whose particlessize is smaller than 0.2 μm and whose surfaces are provided withhydrophobic property by a silane coupling agent or silicone oil to ahydrophobic degree of 40% or above. When the hydrophobic degree is lessthan 40%, moisture is apt to act on the surfaces of the particles in ahot and humid atmosphere, causing the toner to cohere and, therefore,resulting in irregular images. The hydrophobic degree is measured by themethanol wettability method.

A parting agent may be added to the toner applicable to the presentinvention, if necessary. The parting agent may be, for example, defreefatty acid type carnauba wax, montan-based ester wax or oxidized ricewax. These waxes may be used alone or in combination and shouldpreferably have a concentration of 15 wt % in the entire toner.

The present invention is practicable with any conventional carrier,e.g., iron powder, ferrite powder, nickel powder or similar magneticpowder or glass beads with or without the surface thereof coated withresin.

Examples of the present invention and comparative examples will bedescribed hereinafter.

Referring to FIG. 1, a developing device for practicing the presentinvention is shown and generally designated by the reference numeral 10.As shown, the device 10 has a toner cartridge 12 having an agitator 12atherein. Toner from the cartridge 12 is introduced into a casing 16 viaa toner supply roller 14. An agitator in the form of a roller 18 isdisposed in the casing 16 and agitates the developer while feeding it toa developing roller 20. The toner deposited on the developing roller 20in a layer is brought into contact with the surface of a photoconductivedrum 22, thereby developing a latent image electrostatically formedthereon. A magnetic bridge type concentration sensor 24 is mounted onpart of the outer periphery of the casing 16. There are also shown inthe figure a doctor blade 26, a conveyor screw 28, and a separator 30.As shown in FIG. 2, the sensor 24 has a magnetic bridge section 32having an adjustable ferrite core, a phase detector 34, and a voltagecomparator 36. The reference numeral 38 designates the developer. Asshown in FIG. 2, a primary winding 42 and secondary windings 43a and 43bare wound around a core 41. The secondary windings 43a and 43b areconnected in series such that they generate fluxes opposite in directionto each other. The primary winding 42 is connected to an oscillator 44.A magnetic circuit 45a is held in contact with a developer 38 while amagnetic circuit 45b includes a ferrite core 46 capable of adjusting theflux. In this configuration, as the toner in the developer 38 isconsumed, the effective permeability of the developer 38 and, therefore,the mutual inductance M₁ of the magnetic circuit 45a varies. Theresulting difference between the mutual inductance M₁ and the mutualinductance M₂ of the other magnetic circuit 45b produces a differentialoutput. The ferrite core 46 is positioned such that the mutualinductances M₁ and M₂ are equal to each other when the developer has apreselected toner concentration. Then, a relation M₁ <M₂ will hold ifthe actual toner concentration is higher than the preselected value, ora relation M₁ >M₂ will hold if the former is lower than the latter. Thedifferential output is amplified and then compared with a referencesignal by the phase detector 34. The output of the phase detector 34 iscompared with a reference voltage by the voltage comparator 36. Theoutput of the voltage comparator 36 is used to operate a tonerreplenishing circuit, not shown, for replenishing toner. FIG. 3 is agraph indicating a relation between the output voltage of the sensor 24and the toner concentration.

[Carrier]

In Example 1 and Comparative Example 1, spherical Fe, Cu, Zn of ferritehaving a particle size of about 100 μm was prepared.

In Example 2, the substance used in Example 1 was coated with siliconeresin.

In Examples 3, 4, 5 and 6, spherical Fe, Zn or ferrite having a particlesize of about 70 μm was coated with silicone resin.

[Toner]

In Examples 1, 2 and 3 and Comparative Example 1, 88 parts by weight ofstyrene resin, 10 parts by weight of carbon black and 2 parts by weightof quaternary ammonium salt were melted by heat, dispersed, and thenclassified to produce toner having a mean particle size of about 11 μm.

In Example 4, Example 1 was repeated to produce toner whose meanparticle size was about 12 μm.

In Example 5, 0.5 part by weight of silica was applied to 100 parts byweight of the particles produced in Example 4.

In Example 6, 1.0 part by weight of silica was applied to 100 parts byweight of the particles produced in Example 4.

In Example 7, Example 6 was repeated to produce toner having a meanparticle size of about 8 μm.

In Example 8, Example 6 was repeated to produce toner having a meanparticle size of about 7 μm.

FIG. 3 shows the output of the sensor 24 shown in FIG. 2 and used inrelation to the above Examples and Comparative Examples. The meanroughness along the center line is 0.25 μm Ra.

Developers having characteristics listed in Table 1 below were used tomeasure the output of the sensor 24 when the toner concentration was 1%,2%, and 3%. Deviations from ideal output voltages are represented by ΔV;"3.2 V", "2.5 V" and "1.8 V" shown in Table 1 are the ideal outputvoltages.

                                      TABLE 1                                     __________________________________________________________________________                  Developer           Sensor Output                                      Mean   Bulk Density                                                                          Carrier                                                                            Toner  Voltage (V)  Δ(V)                                                                              Toner                       Roughness                                                                            for 2 Wt % of                                                                         Bulk Cohesion                                                                             1%   2%  3%  1%  2% 3% Particle                    (μmRa)                                                                            Toner   Density                                                                            Degree (%)                                                                           3.2 V                                                                              2.5 V                                                                             1.8 V                                                                             --  -- -- Size                 __________________________________________________________________________                                                             (μm)              Comp. Ex. 1                                                                          0.60   3.7     4.5  35     3.9  3.4 2.5 +0.7                                                                              +0.9                                                                             +0.7                                                                             11                   Ex. 1  0.25   3.7     4.5  35     3.5  3.8 2.2 +0.3                                                                              +0.3                                                                             +0.4                                                                             11                   Ex. 2  0.25   1.2     3.5  35     3.3  2.7 2.2 +0.1                                                                              +0.2                                                                             +0.4                                                                             11                   Ex. 3  0.25   1.9     2.7  35     3.3  2.4 1.9 +0.1                                                                              -0.1                                                                             +0.1                                                                             11                   Ex. 4  0.25   1.9     2.7  20     3.2  2.6 1.7 0.0 +0.1                                                                             -0.1                                                                             12                   Ex. 5  0.25   1.9     2.7  15     3.1  2.6 1.8 -0.1                                                                              +0.1                                                                             0.0                                                                              12                   Ex. 6  0.25   1.9     2.7   8     3.2  2.5 1.9 0.0 0.0                                                                              +0.1                                                                             11                   Ex. 7  0.25   2.0     2.7  15     3.3  2.5 2.0 +0.1                                                                              0.0                                                                              +0.2                                                                              8                   Ex. 8  0.25   2.0     2.7  25     3.4  2.6 1.9 +0.2                                                                              +0.1                                                                             +0.1                                                                              7                   __________________________________________________________________________

In summary, it will be seen that the present invention is capable ofmeasuring the toner concentration of a two-component developer withaccuracy by using a magnetic bridge type concentration sensor.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. A method of measuring a toner concentration of atwo-component developer, comprising the steps of:(a) passing saidtwo-component developer through a sensor means of a magnetic bridge typeconcentration sensor, a surface of said sensor means having a centerline mean roughness of less than 0.4 μm Ra; (b) sensing a magneticpermeability of said two-component developer via said sensor means; and(c) measuring a toner concentration of said two-component developer inaccordance with the magnetic permeability sensed by said sensor means.2. A method according to claim 1, wherein said two-component developercomprises toner and carrier, said toner having a mean volume particlesize ranging from 5 μm to 10 μm.
 3. A method according to claim 2,wherein said two-component developer has a volume density of 1.9 g/cm³to 2.3 g/cm³ when the toner concentration is 2.0 wt. %.
 4. A methodaccording to claim 2, wherein said carrier has a volume density of 1.5g/cm³ to 3.1 g/cm³.
 5. A method according to claim 2, wherein said tonerhas a cohesion degree in a range of 10% to 30%.
 6. An apparatus formeasuring a toner concentration of a two-component developer,comprising:a magnetic bridge type concentration sensor for sensing amagnetic permeability of said two-component developer by passing saidtwo-component developer through a sensor means thereof, said sensormeans having a surface whose center line mean roughness is less than 0.4μm Ra; and measuring means, connected to said sensor means, formeasuring a toner concentration on the basis of said magneticpermeability sensed by said sensor means.